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Why are our cities sinking and what does this mean for the future?

The world’s cities are at risk of collapsing under their own weight, according to a new study.

New research shows that the growth of large urban areas exerts increased pressure on the Earth’s surface.

Geophysicist Tom Parsons, from the United States Geological Survey (USGS), chose San Francisco as a case study to measure how and why these areas are sinking.

San Francisco bay region has over 7.7 million inhabitants and is the cultural, commercial, and financial centre of Northern California. Parsons discovered that the city may have sunk by 80 millimetres (3.1 inches) as it has grown substantially over time. This is called the level of subsidence (the sudden sinking or gradual downward settling of the ground's surface).

But in some areas of the region there were even more extreme results. The Millennium Tower in San Francisco has sunk by more than 400 millimetres (15.7 inches) over the last decade.

Subsidence is relevant in modelling climate change risk because of rising sea levels, which are a result of our planet warming up. There is already concern that cities are affected by sea levels creeping up to reach them at ground level, but now they are descending themselves.

The Bay Area is under threat from as much as 300 millimetres of sea level rise by 2050, making scientists and residents fearful for its future.

It is “virtually impossible” to calculate the exact weight of a city, says Parsons, but you can hazard an educated guess based on certain factors.

“I approximate the weight of urbanisation by assuming that buildings and their contents make up the majority of it,” he says. All public buildings, parking garages, residential/commercial buildings, light and heavy industrial buildings, warehouses, transportation centres, etc. are part of the built environment.

Then there are the people.

"As global populations move disproportionately toward the coasts, this additional subsidence in combination with expected sea level rise may exacerbate risk associated with inundation," states Parsons in the paper.

He concludes that the weight of San Francisco is around 1.6 trillion kilograms, or the equivalent of 250,000,000 elephants.

This heavy burden is enough to potentially bend the area’s lithosphere, which is the rigid outer part of the Earth consisting of the crust and upper mantle, causing it to sink.

Indicating that his study could be applied to any city in the world, Parsons adds, “the specific results found for the San Francisco Bay Area are likely to apply to any major urban centre, though with varying importance."

Although other causes of subsidence must also be taken into account, including tectonic plate shifting and groundwater pumping, these findings are significant.

Scientists will now factor in city weight into their considerations when calculating how geography may change over time and the areas at risk of rising sea levels.

Findings could be further improved in future with the addition of satellite photos to better analyse the Earth’s surface and predict where likely flood zones might occur, concludes Parsons.

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